base: steal Chromium's NoDestructor.

Pillage from Chromium a wrapper type that skips destruction of its
wrapped type, to avoid problems with premature destruction of
variables with static lifetime.

Test: libbase_test on host
Change-Id: I7d4541f7b59f467b232d5c4f8250dc1ea45e28fa

base/Android.bp

@@ -148,6 +148,7 @@ cc_test {
         "logging_test.cpp",
         "macros_test.cpp",
         "mapped_file_test.cpp",
+        "no_destructor_test.cpp",
         "parsedouble_test.cpp",
         "parseint_test.cpp",
         "parsenetaddress_test.cpp",

base/include/android-base/no_destructor.h

@@ -0,0 +1,94 @@
+#pragma once
+
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <utility>
+
+#include "android-base/macros.h"
+
+namespace android {
+namespace base {
+
+// A wrapper that makes it easy to create an object of type T with static
+// storage duration that:
+// - is only constructed on first access
+// - never invokes the destructor
+// in order to satisfy the styleguide ban on global constructors and
+// destructors.
+//
+// Runtime constant example:
+// const std::string& GetLineSeparator() {
+//  // Forwards to std::string(size_t, char, const Allocator&) constructor.
+//   static const base::NoDestructor<std::string> s(5, '-');
+//   return *s;
+// }
+//
+// More complex initialization with a lambda:
+// const std::string& GetSessionNonce() {
+//   static const base::NoDestructor<std::string> nonce([] {
+//     std::string s(16);
+//     crypto::RandString(s.data(), s.size());
+//     return s;
+//   }());
+//   return *nonce;
+// }
+//
+// NoDestructor<T> stores the object inline, so it also avoids a pointer
+// indirection and a malloc. Also note that since C++11 static local variable
+// initialization is thread-safe and so is this pattern. Code should prefer to
+// use NoDestructor<T> over:
+// - A function scoped static T* or T& that is dynamically initialized.
+// - A global base::LazyInstance<T>.
+//
+// Note that since the destructor is never run, this *will* leak memory if used
+// as a stack or member variable. Furthermore, a NoDestructor<T> should never
+// have global scope as that may require a static initializer.
+template <typename T>
+class NoDestructor {
+ public:
+  // Not constexpr; just write static constexpr T x = ...; if the value should
+  // be a constexpr.
+  template <typename... Args>
+  explicit NoDestructor(Args&&... args) {
+    new (storage_) T(std::forward<Args>(args)...);
+  }
+
+  // Allows copy and move construction of the contained type, to allow
+  // construction from an initializer list, e.g. for std::vector.
+  explicit NoDestructor(const T& x) { new (storage_) T(x); }
+  explicit NoDestructor(T&& x) { new (storage_) T(std::move(x)); }
+
+  NoDestructor(const NoDestructor&) = delete;
+  NoDestructor& operator=(const NoDestructor&) = delete;
+
+  ~NoDestructor() = default;
+
+  const T& operator*() const { return *get(); }
+  T& operator*() { return *get(); }
+
+  const T* operator->() const { return get(); }
+  T* operator->() { return get(); }
+
+  const T* get() const { return reinterpret_cast<const T*>(storage_); }
+  T* get() { return reinterpret_cast<T*>(storage_); }
+
+ private:
+  alignas(T) char storage_[sizeof(T)];
+};
+
+}  // namespace base
+}  // namespace android

base/no_destructor_test.cpp

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "android-base/no_destructor.h"
+
+#include <gtest/gtest.h>
+
+struct __attribute__((packed)) Bomb {
+  Bomb() : magic_(123) {}
+
+  ~Bomb() { exit(42); }
+
+  int get() const { return magic_; }
+
+ private:
+  [[maybe_unused]] char padding_;
+  int magic_;
+};
+
+TEST(no_destructor, bomb) {
+  ASSERT_EXIT(({
+                {
+                  Bomb b;
+                  if (b.get() != 123) exit(1);
+                }
+
+                exit(0);
+              }),
+              ::testing::ExitedWithCode(42), "");
+}
+
+TEST(no_destructor, defused) {
+  ASSERT_EXIT(({
+                {
+                  android::base::NoDestructor<Bomb> b;
+                  if (b->get() != 123) exit(1);
+                }
+
+                exit(0);
+              }),
+              ::testing::ExitedWithCode(0), "");
+}
+
+TEST(no_destructor, operators) {
+  android::base::NoDestructor<Bomb> b;
+  const android::base::NoDestructor<Bomb>& c = b;
+  ASSERT_EQ(123, b.get()->get());
+  ASSERT_EQ(123, b->get());
+  ASSERT_EQ(123, (*b).get());
+  ASSERT_EQ(123, c.get()->get());
+  ASSERT_EQ(123, c->get());
+  ASSERT_EQ(123, (*c).get());
+}